Thyroid disease occurs when the thyroid gland doesn't supply the proper amount of hormones needed by the body. If the thyroid is overactive, it releases too much thyroid hormone into the bloodstream, resulting in hyperthyroidism. ("Hyper" is from the Greek, meaning "over" or "above.") Hyperthyroidism causes the body to use up energy more quickly than it should, and chemical activity (like metabolism) in the cells speeds up.

An underactive thyroid produces too little thyroid hormone, resulting in hypothyroidism. ("Hypo" means "under" or "below.") When the amount of hormone released into the bloodstream is below normal, the body uses up energy more slowly, and chemical activity (metabolism) in the cells slows down.

Although they are two different conditions, in both hypothyroidism and hyperthyroidism the thyroid can become larger than normal. An enlarged thyroid gland is a lump that can be felt under the skin at the front of the neck. When it is large enough to see easily, it's called a goiter. People who don't get enough iodine in their diets also can get an enlarged thyroid, but this is rare in the United States because foods here usually supply enough iodine.